1. Field of the Invention
The present invention relates to semiconductors exhibiting room-temperature ferromagnetism.
2. Related Art
Solid materials are either crystalline or amorphous. A crystalline solid is one in which the atomic arrangement is regularly repeated, and which is likely to exhibit an external morphology of planes making characteristic angles with each other. In many materials, there are actually a variety of solid phases, each corresponding to a unique crystal structure. These varying crystal phases of the same substance are called “allotropes” or “polymorphs”. The mechanical, thermal, optical, electronic, and magnetic properties of crystals are strongly influenced by the periodic arrangement of their atomic cores. A nanoscale particle is a particle having a measurement of 100 nm or less in at least one direction.
A crystal may be regarded as a three-dimensional diffraction grating for energetic electromagnetic waves (typically X-rays) of a wavelength comparable with the atomic spacing; the diffraction pattern will provide information about the periodic arrangements of the atoms. Constructive interference of the electromagnetic waves may occur where the following minimum condition (called the Bragg equation) is satisfied:2d sin θ=nλwhere d is the spacing between crystalline planes, θ is the angle of incidence between the beam of X-rays and the parallel crystalline planes, n is an integer, and λ is the wavelength of the X-rays. This equation will not be satisfied for most angles θ; to investigate crystals of unknown orientation, the rotating crystal method is used, wherein θ is varied as a function of time while X-rays of a single wavelength are presented to a single crystal. A cylinder of photographic film records a spot whenever the Bragg condition is fulfilled.
In the Debye-Scherrer technique, instead of using a single X-ray wavelength and a time-dependent angle of incidence, a crystalline sample is presented with every θ simultaneously. This is achieved by using a finely powdered crystalline sample in which the crystalline orientations are random. Rays for which one crystallite or another satisfy the Bragg condition emerge from the sample as a series of cones concentric with the incident beam direction. Thus a photographic plate records a series of concentric circles. The spacing and pattern of these circles is used to determine the atomic structure of the crystal.
Particle-induced x-ray emission (PIXE) is an analytical technique capable of trace element detection sensitivity of a few parts per million. When ions pass through matter, they interact with the electrons in the atoms and occasionally a vacancy is produced by an excited electron. When this occurs in an inner shell, the vacancy is filled by an electron from an outer shell, and an x-ray photon of characteristic energy is emitted. By measuring the energy of the photon, one can determine the atomic number of the element and the amount of the element present that can be extracted from the area under the x-ray peak. For identification and quantification of trace elements, PIXE is 100 times more sensitive than electron micro-analysis systems.
Mossbauer spectroscopy is a spectroscopic technique based on the Mossbauer effect. In its most common form, Mossbauer Absorption Spectroscopy, a solid sample is exposed to a beam of gamma radiation, and a detector measures the intensity of the beam that is transmitted through the sample. The gamma-ray energy is varied by accelerating the gamma-ray source through a range of velocities with a linear motor. The relative motion between the source and the sample results in an energy shift due to the Doppler effect. In the resulting spectra, gamma-ray intensity is plotted as a function of the source velocity. At velocities responding to the resonant energy levels of the sample, some of the gamma-rays are absorbed, resulting in a dip in the measured intensity and a corresponding dip in the spectrum. The number, positions, and intensities of the dips (also called peaks) provide information about the chemical environment of the absorbing nuclei and can be used to characterize the sample.
In x-ray photoelectron spectroscopy, the sample is illuminated with soft x-radiation in an ultrahigh vacuum. The photoelectric effect leads to the production of photoelectrons, the energy spectrum of which can be determined in a beta-ray spectrometer. The difference between the x-ray photon energy, which is known, and the electron energy, which can be measured, results in the binding energy of the orbital from which the electron was expelled. Measurement of the relative areas of the photoelectron peaks allows the composition of the sample to be determined.
In discussing magnetic fields, the relationship between the magnetic field intensity H and the corresponding magnetic induction B is
                    B        =                              μ            0                    ⁡                      (                          H              +              M                        )                                                  =                                                            μ                0                            ⁡                              (                                  1                  +                                      χ                    m                                                  )                                      ⁢            H                    =                                                    μ                0                            ⁢                              μ                m                            ⁢              H                        =                          μ              ⁢                                                          ⁢              H                                          where M is the magnetization, χm is the magnetic susceptibility, μm is the relative permeability, μ is the absolute permeability, and μ0=4π×10−7 H/m is the permeability of free space.
The magnetic response of most solids is dominated by the orientation of permanent dipoles. The response of a magnetic material is usually expressed in terms of either the magnetization M or the magnetic susceptibility χ, whereM=χH andχ=M/H 
A spinning charged particle constitutes a magnetic dipole. The magnetic dipole moment of an electron is attributed to its “spin,” and creates a magnetic field pointing in a direction perpendicular to the plane in which the electron is spinning, as shown in FIG. 1.
There are four different kinds of magnetic behavior which involve permanent dipoles in a solid, namely paramagnetic, antiferromagnetic, ferromagnetic, and ferrimagnetic. The low temperature ordering, if any, of neighboring dipoles, and the consequent behavior of spontaneous magnetization and/or susceptibility results in hysteresis loops (shown in FIG. 3) in ferromagnetic and ferromagnetic materials.
Paramagnetic behavior, shown in FIG. 2(a), occurs when the magnetic moments of the various atoms are uncorrelated in the absence of a magnetic field, and the sum total of the magnetic moments tends toward zero. The dipoles do tend to become aligned in a magnetic field. The magnetic susceptibility follows the Curie Law:χm=C/T where C is the Curie constant of the solid, and T represents the temperature of the solid.
Antiferromagnetic behavior, shown in FIG. 2(b), occurs when the dipoles, or magnetic moments, alternate, causing the sum total of the magnetic moments to tend toward zero. This arrangement is very stable at low temperatures, and the magnetic susceptibility in an applied field is small. When the temperature rises, the efficiency of this dipole-dipole interaction decreases and the magnetic susceptibility increases, until the spins become “free” at the Neel temperature to respond to a field. At even higher temperatures the behavior becomes paramagnetic, and the magnetic susceptibility follows a modified Curie lawχm=C/(T+θ)
A ferromagnetic solid, represented in FIG. 2(c), is ordered with parallel spins below the Curie temperature Tc, which results in a spontaneous magnetization Ms. The magnitude of this bulk polarization decreases to zero at the Curie temperature Tc (which is well below room-temperature in most ferromagnetic solids), and the paramagnetic susceptibility for the disordered spin system at higher temperatures obeys the Curie-Weiss lawχm=C/(T−Tc)
Ferromagnetism involves the cooperative alignment of permanent atomic dipoles, which arise in atoms having unpaired electrons. The strength of each individual dipole is small, but a completely ordered array of such moments produces a large spontaneous magnetization Ms.
The low temperature ordering in a ferrimagnetic material, as shown in FIG. 2(d), is similar to that of an antiferromagnetic material, but the two opposing spin systems have magnetic moments of unequal magnitude, and a net spontaneous magnetization results. This magnetization declines to zero magnitude when the solid is warmed to the Curie point Tf, and the behavior is once again paramagnetic at higher temperatures.
Hysteresis loops demonstrate a phenomenon wherein a material that did not show any magnetization before the application of a magnetic field exhibits remanent magnetization after the applied magnetic field is removed, as shown in FIG. 3. The coercivity Hc is the value of the magnetic field that must be applied to return the magnetization to zero after the magnetization has been caused to reach its saturation value; the remanence Br (Mr in the text below) is the value of the magnetization of the material after the material has been caused to reach its saturation value and then had the applied magnetic field removed. Non-zero values for the coercivity and remanence of a sample imply that the sample is ferromagnetic.
Oxide semiconductors have been used to detect gases. However, these have all been non-magnetic oxide semiconductors. Their electrical and semiconducting properties (determined by electrical resistivity, carrier concentration, and carrier mobibility) vary with oxygen stoichiometry. Oxygen stoichiometry can be changed by passing a reducing or oxidizing gas. Thus, traditionally, monitoring the changes in the electrical properties with the type and flow rates of gases has been used as a sensing method.
Tin Dioxide, SnO2, is an oxide semiconductor with a wide band gap of ˜3.6 eV. When prepared with oxygen vacancies, SnO2 becomes an n-type semiconductor. When doped with iron (Fe) or cobalt (Co), SnO2 remains an n-type semiconductor. Development of room-temperature ferromagnetism (RTFM) in conventional semiconductors is currently attracting intense interest due to their potential use in spintronics applications. However, most traditional transition metal-doped magnetic semiconductor systems exhibit ferromagnetism only at temperatures that are well below room temperature.